Backport commits 720f351a3eea5e5bfa83a6eaf50210cd1fa43992^..0630e4aaa10c3fb8c79c2542a229f5c0632cddde.
At the time of writing, ::gentoo no longer has <dev-python/twisted-23.8.0.
These commits refactor the signal handling logic to support newer versions of Twisted.

Author: Lucio Sauer <watermanpaint@posteo.net>
--- a/scrapy/crawler.py
+++ b/scrapy/crawler.py
@@ -404,8 +404,8 @@ class CrawlerProcess(CrawlerRunner):
         :param bool stop_after_crawl: stop or not the reactor when all
             crawlers have finished
 
-        :param bool install_signal_handlers: whether to install the shutdown
-            handlers (default: True)
+        :param bool install_signal_handlers: whether to install the OS signal
+            handlers from Twisted and Scrapy (default: True)
         """
         from twisted.internet import reactor
 
@@ -416,15 +416,17 @@ class CrawlerProcess(CrawlerRunner):
                 return
             d.addBoth(self._stop_reactor)
 
-        if install_signal_handlers:
-            install_shutdown_handlers(self._signal_shutdown)
         resolver_class = load_object(self.settings["DNS_RESOLVER"])
         resolver = create_instance(resolver_class, self.settings, self, reactor=reactor)
         resolver.install_on_reactor()
         tp = reactor.getThreadPool()
         tp.adjustPoolsize(maxthreads=self.settings.getint("REACTOR_THREADPOOL_MAXSIZE"))
         reactor.addSystemEventTrigger("before", "shutdown", self.stop)
-        reactor.run(installSignalHandlers=False)  # blocking call
+        if install_signal_handlers:
+            reactor.addSystemEventTrigger(
+                "after", "startup", install_shutdown_handlers, self._signal_shutdown
+            )
+        reactor.run(installSignalHandlers=install_signal_handlers)  # blocking call
 
     def _graceful_stop_reactor(self) -> Deferred:
         d = self.stop()
--- a/scrapy/utils/ossignal.py
+++ b/scrapy/utils/ossignal.py
@@ -19,13 +19,10 @@ def install_shutdown_handlers(
     function: SignalHandlerT, override_sigint: bool = True
 ) -> None:
     """Install the given function as a signal handler for all common shutdown
-    signals (such as SIGINT, SIGTERM, etc). If override_sigint is ``False`` the
-    SIGINT handler won't be install if there is already a handler in place
-    (e.g.  Pdb)
+    signals (such as SIGINT, SIGTERM, etc). If ``override_sigint`` is ``False`` the
+    SIGINT handler won't be installed if there is already a handler in place
+    (e.g. Pdb)
     """
-    from twisted.internet import reactor
-
-    reactor._handleSignals()
     signal.signal(signal.SIGTERM, function)
     if signal.getsignal(signal.SIGINT) == signal.default_int_handler or override_sigint:
         signal.signal(signal.SIGINT, function)
--- a/scrapy/utils/testproc.py
+++ b/scrapy/utils/testproc.py
@@ -2,7 +2,7 @@ from __future__ import annotations
 
 import os
 import sys
-from typing import Iterable, Optional, Tuple, cast
+from typing import Iterable, List, Optional, Tuple, cast
 
 from twisted.internet.defer import Deferred
 from twisted.internet.error import ProcessTerminated
@@ -26,14 +26,15 @@ class ProcessTest:
         env = os.environ.copy()
         if settings is not None:
             env["SCRAPY_SETTINGS_MODULE"] = settings
+        assert self.command
         cmd = self.prefix + [self.command] + list(args)
         pp = TestProcessProtocol()
-        pp.deferred.addBoth(self._process_finished, cmd, check_code)
+        pp.deferred.addCallback(self._process_finished, cmd, check_code)
         reactor.spawnProcess(pp, cmd[0], cmd, env=env, path=self.cwd)
         return pp.deferred
 
     def _process_finished(
-        self, pp: TestProcessProtocol, cmd: str, check_code: bool
+        self, pp: TestProcessProtocol, cmd: List[str], check_code: bool
     ) -> Tuple[int, bytes, bytes]:
         if pp.exitcode and check_code:
             msg = f"process {cmd} exit with code {pp.exitcode}"
--- a/setup.py
+++ b/setup.py
@@ -6,8 +6,7 @@ version = (Path(__file__).parent / "scrapy/VERSION").read_text("ascii").strip()
 
 
 install_requires = [
-    # 23.8.0 incompatibility: https://github.com/scrapy/scrapy/issues/6024
-    "Twisted>=18.9.0,<23.8.0",
+    "Twisted>=18.9.0",
     "cryptography>=36.0.0",
     "cssselect>=0.9.1",
     "itemloaders>=1.0.1",